Beam formation unit comprising two axicon lenses, and device comprising one such beam formation unit for introducing radiation energy into a workpiece consisting of a weakly-absorbent material

ABSTRACT

The invention is directed to a beam-shaping unit for generating a beam bundle which is focused in a punctiform manner, propagates in a ring shape, and has a radiationless central area, comprising a focusing lens, a first axicon and a second axicon, and to an arrangement with a beam-shaping unit of this kind for introducing radiation energy into a workpiece comprising weakly absorbent material which is arranged between a first resonator mirror and a second resonator mirror. The first resonator mirror which is arranged in front of the workpiece in the radiating direction is located in the radiationless central area. The radiation energy can be absorbed to the maximum extent by repeatedly passing through the same interaction volume in the workpiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of International Application No.PCT/DE2003/002853, filed Aug. 25, 2003, and German Application No. 10240 033.4, filed Aug. 28, 2002, the complete disclosures of which arehereby incorporated by reference.

a) FIELD OF THE INVENTION

The invention is directed to a beam-shaping unit of the generic typeknown to the person skilled in the art for focusing a beam bundle and toan arrangement of the generic type known from JP 10 244 386 A.

b) DESCRIPTION OF THE RELATED ART

The introduction of high-output electromagnetic radiation into aworkpiece comprising a material which is only weakly absorbent continuesto be a central problem in materials processing. In this connection,laser materials processing in particular assumes a central role incorresponding developments, since there is a series of suitableradiation sources available with characteristics that can be efficientlyadapted to a broad range of applications.

Often, it is not sufficient simply to focus the beam on the surface ofthe workpiece in order to make truly effective use of the availableenergy, i.e., to introduce this energy into the workpiece in an optimalmanner spatially and quantitatively. Accordingly, a large number ofmethods and arrangements have been developed to introduce energy in anoptimal manner adapted to the specific machining tasks. These methodsand arrangements aim at shaping and guiding the beam in different waysdepending substantially on the various machining tasks (e.g., severing,ablating or perforating) and the different geometry and material of theworkpiece to be machined.

Of the many solutions in the prior art, only those which either shapethe beam bundle to form a beam ring and/or guide the beam repeatedlythrough the workpiece are considered relevant to the invention.

In the solutions known from the prior art, a beam ring is formedexclusively for the purpose of a circle-shaped irradiation of aworkpiece surface, e.g., in order to cut out a lens. Ideally, the beamis focused in the shape of a ring on the workpiece surface. It is knownto use an axicon to transform the beam bundle into a beam ring.

For example, in U.S. Pat. No. 4,456,811 (or EP 0 189 027 A1), the laserbeam is shaped into a focused ring by a combined collecting lens/axiconand a conical mirror in such a way that a curved, rotationally-symmetricworkpiece surface is impinged perpendicularly by this focus ring and isefficiently machined.

U.S. Pat. No. 4,623,776 describes a very similar arrangement in whichplastic lenses, for instance, can be cut out in an optimal manner bymeans of the generated focused beam ring.

The basic aim of the axicon/focusing lens combination, namely, togenerate a plane, focused beam ring, is addressed in patents DE PS 28 21883 (U.S. Pat. No. 4,275,288) and U.S. Pat. No. 3,419,321, wherein thegenerated beam ring is used, for example, to cut out holes with adefined diameter, to weld contours of this kind, or the like operationsin laser materials processing.

The only thing that the solutions mentioned above have in common withthe subject matter of the present invention is that a beam bundle isshaped by means of an axicon.

More relevant in terms of objective are those solutions in which stepsare taken to guide the beam bundle through the workpiece repeatedly inorder to increase the energy input, so that materials that are highlytransmissive but only weakly absorbent can also be machined.

For example, GB 2 139 614 A describes an arrangement whose mainobjective is, on the one hand, to shape the laser beam focused on theworkpiece in a specific manner so that a stress crack is formed in adefined manner when cutting glass and, on the other hand, to achieve asecond focused pass through the same interaction volume in the workpieceby means of a second focusing mirror arranged on the back of theworkpiece. The amount of radiation energy absorbed is increased by thedouble passage of the laser beam through the workpiece.

JP Patent No. 10 244 386 A, published in Patent Abstracts of Japan,likewise shows a method for severing workpieces by generating a thermalstress crack. The laser beam is transmitted by the workpiece at leasttwice simultaneously or successively in time along the severing zonesubstantially at the same location or at locations at a slight distancefrom one another. The laser beam passes through a semitransparent mirrorbefore striking the workpiece. The transmitting beam component passesthrough the workpiece, is reflected back into the workpiece by a mirrorarranged underneath the workpiece and again strikes the semitransparentmirror, which reflects a portion of the beam back to the workpiece. Inthis way the beam passes repeatedly through the workpiece which isarranged between this mirror and a second mirror. However, the energyloss is extreme. The beam components that are reflected by thesemitransparent mirror when the beam first impinges and the beamcomponents that are transmitted when impinging again on thesemitransparent mirror are wasted in terms of machining the workpiece inthe first place and are reflected back into the radiation source in thesecond place.

There is no solution known from the prior art by which the availableradiation energy can be absorbed nearly completely in a smallinteraction volume of a partially transparent workpiece. This fact isexplained particularly in that an optical element which reflects thebeam back into the interaction volume at least twice for this purpose isalways in the optical beam path between the radiation source and theworkpiece. Since there is no optical element, including optical films,which completely transmits an unchanged beam in one passage directionand reflects it completely in the other passage direction, energy lossis inevitable. It is clear to the person skilled in the art thatpractically any optical element located in a beam path will alwaysresult in losses, even if these losses are minor. There is a high losswhenever the element is at times reflective and at times transmissivefor radiation coming from different directions as, for example, in asemitransparent mirror or a splitter cube.

OBJECT AND SUMMARY OF THE INVENTION

It is the primary object of the invention to provide a beam-shaping unitwhich shapes a ring-shaped beam bundle that can be focused in apunctiform manner and that has a radiationless central area in whichthere can be arranged an optical element that will not be affected by abeam bundle coming from a radiation source and penetrating thebeam-shaping unit and which therefore does not cause losses.

It is a further object of the invention to provide an arrangement bywhich, through the use of a beam-shaping unit that meets the object ofthe invention, relatively weakly absorbent, extensively transparentmaterials can be machined by electromagnetic radiation, preferably theradiation of a high-power laser, in such a way that ultimately the totalabsorption in the material is high and, therefore, the radiation energyis made use of efficiently for the machining process throughmultiplication of the individual absorbencies, and it is ensured thatthe radiation energy is introduced into the smallest possibleinteraction volume (volume of the workpiece over which the radiation isabsorbed) in order to achieve high-precision machining, and radiationthat is not absorbed is substantially prevented from being coupled backinto the radiation source.

The object, according to the invention, is met for a beam-shaping unitfor generating a ring-shaped beam bundle with a focusing lens followedby a first axicon which is arranged with the focusing lens on a commonoptical axis and whose conical optical surfaces faces the focusing lensand in that a second axicon is arranged on the optical axis in front ofthe focusing lens with its conical optical surface remote of thefocusing lens and the parameters of the focusing lens and of the axiconsand their distance from one another are selected in such a way that abeam bundle coming from a radiation source arranged on the optical axisin front of the second axicon and passing through the beam-shaping unitis shaped into a ring-shaped beam bundle which is focused in a firstfocus point and which subsequently diverges and which has a central areain which radiation is absent.

The object, according to the invention, is further met for anarrangement for introducing radiation energy into a workpiece comprisinga weakly absorbent material, with a radiation source and a resonatorunit, comprising a first resonator mirror and a second resonator mirrorbetween which the workpiece is fixed and in that a beam-shaping unit, asdiscussed immediately above, is arranged in front of the resonator unitand the first resonator mirror is located in the radiationless centralarea behind the first focus point, in that there is a collecting lenswhich surrounds the first resonator mirror and which focuses theincident beam bundle in the workpiece in a second focus point, in thatthe second resonator mirror is located in a radiationless central areabehind the second focus point and surrounded by a focusing mirror whichshapes the divergent beam bundle coming from the second focus point intoa convergent beam bundle and reflects it onto a mirror that is arrangedbetween the focusing mirror and the workpiece and reflects the beambundle on to the second resonator mirror, in that the mirror has a holethat is just large enough to allow the beam bundle coming from thesecond focus point and traveling in the direction of the focusing mirrorto pass through without being influenced and through which the beambundle reflected by the second resonator mirror passes without beinginfluenced and is focused a second time inside the workpiece in a secondfocus point before subsequently striking the first resonator mirror andbeing reflected back by the latter into the workpiece.

Advantageous embodiment forms are described in the subclaims.

Since the nature of the beam-shaping unit according to the inventionpredominantly determines the nature of the arrangement according to theinvention, the nature of the invention will be described in thefollowing with reference to the arrangement.

It is clear to the person skilled in the art that an optical beam canpass through a medium (workpiece) repeatedly on the same optical path(in order to introduce radiation energy into a small interaction volume)only when a reflector is arranged on both sides of the medium, that is,it is mandatory that a first reflector is arranged in front of theworkpiece in the radiating direction. This first reflector has nofunction for the first coupling in of the beam into the workpiece andshould therefore allow the beam to pass as far as possible without beinginfluenced. It should reflect the impinging radiation as completely aspossible only after the beam impinges on the first reflector again afterbeing reflected at the second reflector and after passing through theworkpiece a second time.

Herein lies the fundamental idea of the invention: in order for thefirst reflector not to influence the beam practically at all before itfirst strikes the workpiece, the beam bundle should be shaped in such away that it does not pass through the first reflector, even though thelatter is located in the beam path, but surrounds it instead. This beamshaping is realized by a beam-shaping unit according to the invention.It is substantial to the invention that this beam-shaping unit shapesthe beam bundle coming from the radiation source in such a way that theentire beam is collected in a first focus point in the radiatingdirection behind this beam-shaping unit and subsequently diverges insuch a way that it propagates as a ring with a defined radiationlesscentral area whose diameter increases linearly with the distance fromthe first focus point. A first resonator mirror belonging to a resonatorunit arranged downstream of the beam-shaping unit is arranged in thisradiationless central area. The resonator unit shapes and guides thebeam defined by the described propagation characteristics in such a waythat the first focus point is imaged in the workpiece in a second focuspoint and the radiation energy is accordingly concentrated in apunctiform manner in a small interaction volume that is penetratedrepeatedly, but at least four times, by the beam, and the sum of theradiation absorbed by the workpiece is increased in this way to acorresponding multiple of the individual absorption.

In order to collect the beam in a first focus point and subsequentlyexpand it such that a beam ring occurs, it is not sufficient to workwith a focusing lens and only one axicon as is usual in solutions knownfrom the prior art which generate a focus ring on the workpiece surfacein a uniform manner. These solutions are characterized in that theintersection of the central rays of the ring generated by the axicon andthe focus point of the individual beam segments of this ring aredisplaced relative to one another considered in the beam propagatingdirection. When the parameters of the focusing lens, the axiconparameters and their relative spacing are adapted to one another in sucha way that all rays meet at one point, the effect is ultimately that ofan adequate individual lens, i.e., the rays proceeding from this pointdo not form a ring with a central radiationless hole that increases insize in proportion to the distance from the focus. This problem can besolved, according to the invention, by introducing a second axicon intothe beam path. The combined effect of these three components (twoaxicons plus focusing lens) is the following: the first axicon providesfor the desired shaping of the ring. The focusing lens causes the raysto be collected in the known ring focus which degenerates into therequired first focus point when a suitable convergence supplied by thesecond axicon is superimposed. This first focus point is now actuallythe starting point of a beam propagating in a ring-shaped manner with acenter entirely free of radiation. This special beam shaping makes itpossible to realize the actual goal of the arrangement by means of theresonator unit according to the invention. That is, since all beamcomponents proceed from one point, namely, the first focus point, behindthe beam-shaping unit, they can always be “refocused” again, i.e.,collected in another focus point, by the collecting lenses or concavemirrors arranged downstream, and the ring shape is maintained on thebeam path between the focus points. This is the precondition that mustbe met in order for the resonator unit arranged downstream to fulfillits function. The resonator unit is constructed substantially asfollows: The input element of the resonator unit is a collecting lenswhose distance from the first focus point is selected in such a way thatthe radiationless central area of the beam bundle is large enough for aconcave mirror functioning as a first resonator mirror to be arrangedtherein without masking portions of the beam bundle. The beam bundle isaccordingly guided entirely through this collecting lens when enteringthe resonator unit without being influenced by the first resonatormirror. The first collecting lens focuses the beam in the interactionvolume of the workpiece to be machined, for example, a glass plate. Amirror with a central hole is arranged directly behind the workpiecewhich can have a thickness of several millimeters. This central holemust be large enough so that the beam bundle which diverges again behindthe workpiece can pass through without losses. The beam bundle continuesalong the optical path to impinge on a focusing mirror which is arrangedat a distance from the mirror with the central hole such that, on theone hand, there is again a sufficiently large radiationless central areaon the focusing mirror which makes it possible to position another smallconcave mirror that functions as a second resonator mirror and, on theother hand, by means of a suitable selection of the radius of curvatureof the focusing mirror, the beam bundle which, after being reflected atthe focusing mirror, runs back in the direction of the mirror andconverges again is completely reflected by the mirror and is collectedin a third focus point located between the mirror and the secondresonator mirror generally in the vicinity of the second resonatormirror. The beam bundle, now with a relatively small diameter, strikesthe second resonator mirror whose radius of curvature is such that thebeam is focused again, namely, in the interaction volume in theworkpiece. The bundle which is already very narrow after being reflectedat the second resonator mirror now penetrates the workpiece a secondtime and runs in the direction of the first resonator mirror. The radiusof curvature of the first resonator mirror is adapted in such a way thata reflection is carried out anew, this time precisely in the third focuspoint. Therefore, the bundle strikes the second resonator mirror in sucha way that the initial conditions of the “first resonator circuit” arereproduced, but with a reduced bundle diameter. In this way, thearrangement according to the invention works in a manner analogous to aresonator, i.e., the beam passes repeatedly through the interactionvolume in the workpiece. The bundle “dies out” through absorption in theworkpiece without the beam components returning to the radiation source.

The efficiency of the arrangement, i.e., the ratio of radiation energydeposited in the workpiece to the input energy supplied by the radiationsource, is determined only by the inevitable principal losses,particularly the absorption at the optical elements, diffraction losses,and losses due to imaging errors and alignment errors and should be morethan 50% also in relatively weakly absorbent materials (with anabsorption of less than 10% per passage).

With respect to the beam configuration described above, it is taken intoaccount that with every odd-numbered passage of the beam bundle throughthe workpiece following the first passage the beam bundle is not focusedwithin the workpiece (second focus point) and therefore in the actualinteraction volume, but rather in the vicinity of the workpiece (in thethird focus point), since stable resonance behavior between the firstresonator mirror and second resonator mirror can only be achieved inthis way. However, this is not detrimental for the majority ofapplications, particularly also because the bundle diameter decreasesfrom one passage to the next and is on the order of 1 mm or less inevery case, given a suitable dimensioning of the optical parameters. Forcertain tasks, the beam components that are not sharply focused can evenbe very advantageous because they can have a temperature-regulatingeffect or can act to decrease the temperature gradient.

The geometry of the resonator unit can also be modified easily so thatit is possible to adapt optimally to the workpiece characteristics,especially to the absorption behavior and the desired machining results.For example, the focal length of the first resonator mirror can beselected in such a way that the bundle coming from the second resonatormirror is focused in the interaction volume again, that is, practicallyreturns into itself. After reflections at the second resonator mirror,the mirror and the focusing mirror, the workpiece is even penetrated afourth time in a focused manner. When the absorption is not too low(≧20%), the majority of the radiation energy is deposited in the zone tobe machined. However, the residual radiation that is not absorbed in theinteraction volume could then be coupled back into the radiation sourceif special steps are not taken. This problem can be solved relativelyeasily in that a beam decoupling unit that eliminates the returninglight practically completely is placed in the beam path between theradiation source and the beam-shaping unit. However, for efficientoperation of the beam decoupling unit, the radiation must come from theradiation source in a linearly polarized manner. The beam decouplingunit then functions as follows: first, the linearly polarized beamcoming from the radiation source traverses a polarizer which is adjustedfor full transmission, i.e., the beam bundle undergoes only minimallosses. The beam is then transformed into a circularly polarized beam bypassing through a quarter-wave plate. This transformation is required,or at least useful, for numerous applications because the unwanteddirectional dependency of the results of machining disappears whencircularly polarized radiation is used. The circular polarization issubstantially maintained along the further beam path. The returningportion also has this characteristic. It now traverses the beamdecoupling unit in the opposite direction with the result that thequarter-wave plate “further rotates” the polarization vector in such away that linearly polarized light results again from the circularlypolarized light, but with a polarization plane that is rotated by 90°relative to the radiated electromagnetic radiation field. However, thepolarizer is now in the blocking direction for this radiation, i.e., thebeam is prevented from being coupled back into the radiation source.

In the arrangement according to the invention, with any given opticalparameters, the location with the most favorable ratios with respect tointensity, beam diameter and intensity distribution can generally befound and freely selected within relatively wide limits by displacingthe workpiece along the optical axis. This is an important option, e.g.,when severing glass by means of stress cracks. For this application inparticular, it may be necessary to generate a starting crack. For thispurpose, the invention offers the following three possibilities by wayof example:

1. A radiation source is chosen which, in addition to continuous mode,enables a pulse mode with a highly exaggerated output peak (e.g., bymeans of a q-switching laser) in order to cause a deliberate, sensitive“destruction” of the material joint that starts the crack at thebeginning of the machining process with a pulse of this type in thefocus volume. The radiation source is then switched to normal operationand the starting crack is driven through the workpiece as a stress crackin the desired manner, i.e., with the aim of a defined severing process.

2. Since the arrangement according to the invention makes it possible tovary the relative position of the focus and workpiece quickly, it isalso possible, for example, to proceed in such a way that the workpiecelies exactly in the focus as the machining of the workpiece commences(with continuously operating radiation source). With sufficientradiation output, the desired sensitive destruction of the materialjoint, i.e., the starting crack, can be produced. For further machining,in which melting and evaporation are not desirable, the workpiece ismoved out of the focus area until optimal intensity conditions forsevering by means of a stress crack are achieved.

3. A particularly flexible variant results when using an additionalradiation source for generating the starting crack or for a series ofsuch starting points with the aim of a precise contour even withcomplicated workpiece shapes. This additional radiation source shouldpreferably be a pulsed laser. In an advantageous arrangement, adeflecting mirror which is transparent for the actual working beam(coming from the first collecting lens) but which reflects the beam ofthe second laser completely can be arranged, e.g., directly in front ofthe workpiece. The beam of the second laser is sharply focused on theworkpiece by means of a 90-degree deflection at this deflecting mirror.The additional radiation source and the focusing optics for its beam arearranged laterally outside the main beam path. By means of suitableadjusting elements that can also be put into effect during the machiningprocess, e.g., by means of actuating motors, it is possible to adjust awide variety of desired relative positions between this sharp focus andthe actual interaction volume. For example, the direction of the stresscrack can be specifically influenced during the machining process.

When severing glass by means of Nd:YAG lasers or diode lasers, a smallTEA-CO₂ laser, whose radiation is extensively absorbed by glasses, canbe used as an additional radiation source. A pulse from a laser of thiskind that is sharply focused on the surface of the glass sample is thensufficient for generating the desired starting crack.

Additional novel applications that are only mentioned herein but are notdescribed in more detail are made possible by the arrangement accordingto the invention. For example, a nonlinear optical crystal can bearranged in the third focus point and used to generate higher harmonicsof the original beam. High conversion rates are made possible as aresult of the resonator arrangement and the relatively high intensity inthis third focus point. In this way, the workpiece could be machined bythe fundamental wave of the radiation as well as with a higher harmonic,which can lead to advantageous results.

The beam-shaping unit according to the invention has been described verythoroughly in connection with a resonator unit according to theinvention, in which a first resonator mirror is located in theradiationless central area. Other optical elements can also be arrangedin place of this first resonator mirror for other uses of thebeam-shaping unit. For example, a deflecting mirror could be arranged atthis location. This deflecting mirror need no longer be asemitransparent mirror; rather, the impinging beam can be reflected tothe maximum extent by a highly reflective coating. In this way, aplurality of beam bundles can be superimposed without losses.

The invention will be described more fully in the following withreference to embodiment examples shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a beam path through an axicon (prior art);

FIG. 2 shows a beam path through a collecting lens and an axicon (priorart);

FIG. 3 shows a beam path through a beam-shaping unit comprising twoaxicons and a collecting lens;

FIG. 4 shows the propagation of the beam cone behind a beam-shaping unitaccording to FIG. 3;

FIG. 5 shows a resonator unit and detailed beam path;

FIG. 6 shows an entire arrangement with radiation source, beam-shapingunit and resonator unit;

FIG. 7 shows a decoupling unit; and

FIG. 8 shows a resonator unit with an additional radiation source forgenerating starting cracks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An arrangement according to the invention substantially comprises aradiation source 6, a beam-shaping unit 4 and a resonator unit 5. Thetwo axicons that are used in the beam-shaping unit 4 take on a centralfunction. Therefore, to facilitate understanding of the invention, FIG.1 first illustrates the basic function of an individual axicon 1 whichis characterized by the cone angle δ. The incident beam bundle, which isassumed to be slightly divergent and symmetric with respect to rotation,e.g., with a Gaussian or rectangular (top-hat) intensity cross section,impinges on the beam-shaping side (conical surface) of the first axicon1 orthogonally and so as to be exactly centered. As a result of therefraction at its conical surface, the beam exits the first axicon 1 asa divergent ring. The central beams (shown as a dashed line) intersectat a point lying on the optical axis 9 (shown as a dash-dot line); thesymmetry of the beam bundle with respect to rotation must always betaken into account.

FIG. 2 shows an arrangement for a typical use of a first axicon 1 knownfrom the prior art, that is, for generating a ring focus. For thispurpose, a focusing lens 2 which forms a convergent beam bundle from abeam bundle such as that shown striking the first axicon 1 in FIG. 1 isarranged in front of the first axicon 1. Due to the effect of the firstaxicon 1, which was already described with reference to FIG. 1, aconvergent ring beam results after the beam passes through this firstaxicon 1. The central rays of this ring beam intersect again at a pointon the optical axis 9 of the first axicon 1. A ring focus or focussedbeam ring 10 results in a defined plane corresponding to the focallength of the focusing lens 2. Regardless of the parameters of thefocusing lens 2 and those of the first axicon 1, the intersection of thecentral rays and of the ring focus are always at a distance a from oneanother.

However, it is necessary for the function of the arrangement accordingto the invention that a approaches zero, which only means that thefocused beam ring 10 degenerates into a focus point that coincides withthe intersection of the central rays. This first requirement, inconnection with the second requirement that the beam reoccurs as adivergent ring with a radiationless central area after this focus point,can now be achieved by means of the inventive arrangement of a secondaxicon 3 in front as is shown in FIG. 3.

The first axicon 1, the focusing lens 2 and the second axicon 3 togetherform the beam-shaping unit 4 which is arranged on a common optical axis9 with a resonator unit 5, described later on, and a radiation source 6and determines the arrangement according to the invention. The overallarrangement is shown in FIG. 6, but is described with respect to itsessential components, the beam-shaping unit 4 in FIG. 3 and theresonator unit 5 in FIG. 5.

The second axicon 3 with a cone angle δ₂ ensures that the incident beambundle, as was described in detail in FIG. 1, is initially pre-shaped insuch a way that it already impinges on the focusing lens 2 in the shapeof a ring so that the beam bundle which is convergent following thefocusing lens 2 already impinges on the first axicon 1 in the shape of aring with a radiationless central area and is finally interrupted by theeffect of the latter in such a way that the entire convergentring-shaped beam bundle is focused in a first focus point 8 at a defineddistance behind the first axicon 1 and subsequently opens as a divergentbeam ring 10 with an increasingly larger radiationless central area. Thewedge angle δ₁ of the first axicon 1 and δ₂ of the second axicon 3 andtheir distances relative to one another and to the focusing lens 2 arebrought into accord with the focal length of the focusing lens 2 in sucha way that the desired beam path results.

The propagation of the beam bundle after the common intersection of allrays in the first focus point 8 is shown once again in FIG. 4. As wasalready shown, the first focus point 8 is the starting point of a beambundle which propagates in a ring-shaped manner with a divergence. Incontrast to the drawings described above, in which the beam bundles areshown in longitudinal section through the optical system by central andmarginal rays, FIG. 4 shows the ring-shaped beam bundle in cross sectionorthogonal to the optical axis 9. The beam bundle is shown in this crosssection as a beam ring 10 with a radiationless central area.

FIG. 5 shows the basic construction of the resonator unit 5 comprising acollecting lens 12, a first resonator 13, a mirror 17 with a hole 16, afocusing mirror 18 and a second resonator mirror 19. The arrangement ofthe elements relative to one another and the operation of the resonatorunit 5 will be described.

The resonator unit 5 is arranged downstream of the beam-shaping unit 4in the radiating direction in such a way that its first subassembly,comprising a collecting lens 12 and a first resonator mirror 13, is at adistance from the first focus point 8 such that the beam ring 10 liescompletely on the collecting lens 12 and the first resonator mirror 13lies completely in the radiationless central area. The collecting lens12 and the first resonator mirror 13 need not necessarily lie in a plane(the person skilled in the art will understand that the principal planesare meant whenever it is stated that optically imaging elements lie in aplane). The collecting lens 12 focuses the ring-shaped beam bundle onthe surface of a workpiece 14 and in the volume of a workpiece 14(interaction volume) in the second focus point 15; the optimal positionof the second focus point 15 relative to the workpiece 14 depends uponthe task at hand. After this first passage through the workpiece andtherefore after the first partial absorption, the beam bundle passesthrough a hole 16 in the mirror 17 which is precisely large enough toallow the beam bundle, which is now divergent again, to pass throughwithout losses. In order to keep this hole 16 in the mirror 17 as smallas possible, the mirror 17 is placed at a short distance behind theworkpiece 14.

As it continues along its path, the beam bundle strikes the focusingmirror 18 which is located at a sufficient distance from the workpiece14 behind the second focus point 15 so that a radiationless central areawith a diameter of sufficient size to position the second resonatormirror 19 therein is also provided in this case. This second resonatormirror 19 can either be arranged so as to be stationary at the focusingmirror 18 or can be freely adjustable in order to ensure additionaldegrees of freedom of the arrangement with resect to adjustment. In thelatter case, the focusing mirror 18 must have a sufficiently largeopening so that the second resonator mirror 19 can, if needed, be tiltedor also displaced along the optical axis 9 by means of an appropriateadjusting unit 21 which is arranged behind the focusing mirror 18, i.e.,outside the entire beam path and, consequently, is not influenced by thelatter. This enables a sensitive adjustment of the actual resonatorwhich is formed by the first resonator mirror 13 and the secondresonator mirror 19.

The continuing beam path following the reflection at the focusing mirror18 makes clear the important function of the mirror 17. It paves the wayfor the function of the resonator insofar as it deflects the beambundle, which now comes from the focusing mirror 18 so as to bedivergent again, into the third focus point 20. It should be noted thatthe mirror 17 need not necessarily be a plane mirror as illustrated, butif necessary can also have an optically active curvature, e.g., when itis desirable to distribute the required collecting action unifying theradiation in the third focus point 20 approximately uniformly to themirror 17 and to the focusing mirror 18. The third focus point 20 whichis located at a relatively short distance in front of the secondresonator mirror 19 plays a central role for the resonator. The focallength of the second resonator mirror 19 and its distance from the thirdfocus point 20 are adapted to one another in such a way that the beambundle is focused a second time in the interaction volume in theworkpiece 14 and strikes the first resonator mirror 13 as a narrow beambundle after the second partial absorption. When the focal length isselected in such a way that the reflected beam is again collectedexactly in the third focus point 20 after the third passage through theworkpiece 14, the condition for a genuine resonator function, i.e., thatthe beam passes back and forth frequently between the two resonatormirrors 13 and 19, is met. As a result of the absorption taking placeduring every passage through the workpiece, the bundle finally “diesout” and the available radiation output is deposited substantially inthe material to be machined, even with weak absorption.

A very substantial second advantage consists in that beam components arecompletely prevented from running back into the radiation source 6, sothat special measures for decoupling the radiation source 6 can beomitted in this preferred construction of the arrangement according tothe invention.

In a second embodiment example for an arrangement according to theinvention, the beam bundle is focused in the interaction volume duringevery passage through the workpiece 14 in order to reduce theinteraction volume and introduce the energy in a more concentratedmanner spatially. For this purpose, the focal length of the firstresonator mirror 13 must be selected in such a way that the beam bundleis not focused in the third focus point 20, but rather a third time inthe workpiece 14 in the second focus point. However, this advantage isgained at a cost in that the residual radiation that is not absorbedruns back in the direction of the radiation source 6 (fourth focusedpassage through the workpiece 14), since the basic condition for theresonator function is no longer met. Accordingly, in this case a specialdecoupling unit 22 must be placed behind the radiation source 6 in theradiation direction as is shown, for example, in FIG. 7.

The beam decoupling unit 22 shown herein requires that the radiationsource 6 transmits linearly polarized electromagnetic radiation, whichapplies for most lasers. In the illustration shown in FIG. 7, it isassumed that its polarization plane lies in the drawing plane. Thisradiation passes through the polarizer 23, which is adjusted forpassing, without substantial losses and subsequently impinges on thequarter-wave plate 24 arranged directly behind the polarizer 23. Thisinfluences the polarization in such a way that a circularly polarizedbeam bundle running in the direction of the continuing beam path resultsfrom the originally linearly polarized beam bundle. The beam bundlereturning after passing repeatedly through the workpiece 14 still hassubstantially the same polarization characteristics, i.e., it is alsostill circularly polarized. When this beam bundle strikes thequarter-wave plate 24 in the opposite direction of travel, thepolarization state is changed in such a way that the circularlypolarized beam becomes linearly polarized again, but with a polarizationplane that is rotated by 90° relative to the original beam bundle.However, for radiation that is polarized perpendicular to the drawingplane, the polarizer 23 lies in the blocking direction, so that thisradiation is prevented from passing and cannot be coupled into theradiation source 6.

A third embodiment example will be described in more detail withreference to FIG. 8. In contrast to the first embodiment example shownin FIG. 6, a deflecting unit 25 is introduced between the collectinglens 12 and the workpiece 14. The radiation from an additional radiationsource 27 which is focused by a lens 26 is coupled into the beam path ofthe total arrangement by means of this deflecting unit 25. Thisadditional radiation is provided for generating a starting crack whensevering glass. The additional radiation source 27 is preferably aTEA-CO₂ laser whose pulsed radiation, with a high pulse output and highpulse energy, is sharply focused on the workpiece 14 by the lens 26. Thedeflecting unit 25 ensures that this additional focus 28 lies exactly atthe desired location relative to the position of the interaction volume.In order to adjust any desired positions for the additional focus 28,the deflecting unit 25 is coupled with an adjusting device 29. Thedeflecting unit 25 can advantageously comprise a plane-parallel platewhich is coated in such a way that it is completely transparent for thewavelength of the incident electromagnetic radiation of the radiationsource 6 and fully reflects the beam of the additional radiation source27.

The deflecting unit 25 can also be a simple mirror 17 for the beam ofthe additional radiation source 27 and is slid into or folded into thebeam path for generating a starting crack and removed again from thebeam path prior to the actual machining of the workpiece 14 which, inthis case where a starting crack is generated, involves severing.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made herein without departing from the true spirit andscope of the present

REFERENCE NUMBERS

-   1 first axicon-   2 focusing lens-   3 second axicon-   4 beam-shaping unit-   5 resonator unit-   6 radiation source-   8 first focus point-   9 optical axis-   10 beam ring-   12 collecting lens-   13 first resonator mirror-   14 workpiece-   15 second focus point-   16 hole-   17 mirror-   18 focusing mirror-   19 second resonator mirror-   20 third focus point-   21 adjusting unit-   22 beam decoupling unit-   23 polarizer-   24 quarter-wave plate-   25 deflecting unit-   26 lens-   27 additional radiation source-   28 additional focus-   29 adjusting device-   δ1 cone angle of the first axicon-   δ2 cone angle of the second axicon-   a distance

1-6. (canceled)
 7. (canceled)
 8. An arrangement for introducingradiation energy into a workpiece made from a weakly absorbent materialcomprising: a radiation source and a resonator unit, wherein theresonator unit comprises a first resonator mirror and a second resonatormirror between which the workpiece is fixed; a beam-shaping unit shapinga ring-shaped beam bundle which has a central radiationless area andarranged in front of the resonator unit and the first resonator mirrorbeing located in a radiationless central area behind a first focuspoint; a collecting lens being provided which surrounds the firstresonator mirror and which focuses an incident beam bundle in theworkpiece in a second focus point; said second resonator mirror beinglocated in the radiationless central area behind the second focus pointand surrounded by a focusing mirror which shapes a divergent beam bundlecoming from the second focus point into a convergent beam bundle andreflects it onto a mirror that is arranged between the focusing mirrorand the workpiece and reflects the beam bundle onto the second resonatormirror; said mirror having a hole that is just large enough to allow thebeam bundle coming from the second focus point and traveling in thedirection of the focusing mirror to pass through without beinginfluenced and through which the beam bundle reflected by the secondresonator mirror passes without being influenced and being focused asecond time inside the workpiece in a second focus point beforesubsequently striking the first resonator mirror and being reflectedback by the first resonator mirror into the workpiece.
 9. An arrangementaccording to claim 8, wherein the parameters of the second resonatormirror and of the focusing mirror are selected in such a way that,before striking the second resonator mirror, the beam bundle reflectedby the focusing mirror is focused in a third focus point that is imagedin the second focus point by the second resonator mirror, and in thatthe parameters of the first resonator mirror are selected in such a waythat a beam bundle coming from the second focus point and impinging onthe first resonator mirror is focused in the third focus point.
 10. Anarrangement according to claim 8 wherein the parameters of the secondresonator mirror and of the focusing mirror are selected in such a waythat the beam bundle reflected by the focusing mirror is focused in thesecond focus point selectively with or without intermediate focusing asecond time after reflection at the second resonator mirror, a thirdtime after reflection at the first resonator mirror, and a fourth timeafter a repeated reflection at the second resonator mirror, and a beamdecoupling unit is arranged in front between the radiation source andthe beam-shaping unit and prevents beam components from being coupledback to the radiation source.
 11. An arrangement according to claim 9,wherein the radiation source emits linearly polarized light and the beamdecoupling unit is formed by a polarizer and a quarter-wave plate. 12.An arrangement according to claim 8, wherein an additional radiationsource is provided for generating a starting crack and the additionalbeam of this additional radiation source can be focused occasionally inthe workpiece by a lens and a deflecting unit.
 13. An arrangementaccording to claim 8, wherein the beam shaping unit comprises: afocusing lens followed by a first axicon which is arranged with thefocusing lens on a common optical axis and whose conical optical surfacefaces the focusing lens; a second axicon being arranged on the opticalaxis in front of the focusing lens with its conical optical surfaceremote of the focusing lens and the parameters of the focusing lens andof the axicons, and their distances from one another being selected insuch a way that a beam bundle coming from a radiation source arranged onthe optical axis in front of the second axicon and passing through thebeam-shaping unit is shaped into a ring-shaped beam bundle which isfocused in a first focus point and which subsequently diverges and whichhas a central radiationless area.